In a statement on its corporate website, the parent company FriendFinder Networks said it "has just been made aware of a potential data security issue and understands and fully appreciates the seriousness of the issue."

"We have already begun working closely with law enforcement and have launched a comprehensive investigation with the help of leading third-party forensics expert."

The data breach was first reported a month ago by security researcher Bev Robb who discovered the data on a "dark Web" site. But the name of the dating site was not disclosed before Friday's report.

"There is a ton of personally identifiable information sitting in a forum on the Darknet," Robb noted last month. "Though the files were stripped of credit card data, it is still relatively easy to connect the dots and identify thousands upon thousands of users who subscribe to this adult site."

Security experts say such stolen data can be used for various email scams and potentially for blackmail. Security researcher Graham Cluley said in a blog post Friday that the website has an obligation to inform its 63 million members worldwide.

"How about posting a warning on your website so your members can look out for phishing emails, or malware which might be sent to them?" Cluley said.

"What about some advice about the type of threats that users could be exposed to -- including, potentially, blackmail -- if their membership of the site is uncovered?"